1. Field of the Invention
The present invention relates to a drying apparatus and a conveying system.
2. Description of the Related Art
Inkjet printers that discharge liquid droplets of ink or the like (ink droplets) from nozzles of a liquid discharge head to record images, characters, and the like on recording media such as printing paper are conventionally known. The inkjet printer includes a conveying path for conveying a recording medium as a conveyed object. A plurality of types of inkjet printers are known; examples thereof include serial printers and line printers. The serial printer, in a conveying path, discharges ink while moving a recording head in a main-scanning direction, which is a direction orthogonal to the conveying direction of a conveyed object, and combines conveying operation in a sub-scanning direction, which is the conveying direction of the conveyed object, to execute printing operation on a recording medium. The line printer has a line head in which recording heads are arranged side by side in the main-scanning direction to perform printing in a print width conforming to the width dimension of a recording medium. Printing operation is executed while moving a conveyed object relatively with respect to this line head.
Inkjet printers in recent years finely control the discharge timing of ink droplets and the size of ink droplets to promote the enhancement of printing image quality and the speed of printing operation. However, because of being a system that drops liquid droplets (liquid) onto a recording medium to form an image, as a problem associated with the enhancement of image quality and speed, a problem in which a liquid droplet adhering to the recording medium moves on the surface of the recording medium and mixes with an adjacent liquid droplet is known. In the case of a recording medium having a coat layer or the like in particular, the liquid droplets are difficult to permeate the recording medium, and color mixture with adjacent ink droplets having different colors is likely to occur. If color mixture occurs, a phenomenon of color bleeding, which makes color boundaries blurred, is likely to occur. If color bleeding occurs in a print result, the sharpness of a print image and the like degrade. In other words, image quality degrades.
Owing to the enhancement of the speed of printing operation, if the recording medium is sent to posttreatment of the printing operation with the permeation of ink into the recording medium and the drying of the ink droplets adhering to the recording medium remaining insufficient, problems are likely to occur such as the occurrence of ink transfer (picking) from the recording medium to conveying rollers and the transfer of ink (blocking) between recording media stacked in a conveying path. Examples of known techniques as measures against picking and blocking include applying a powdery material that prevents ink transfer after printing and arranging a drying apparatus in the downstream process of a printing unit to dry ink on the print face of the recording medium.
Among these techniques, a system that arranges a heating roller incorporating a heating source within conveying rollers forming a conveying path is known as the technique of drying ink. The system using the heating roller is a system that brings a recording medium as an object to be dried into contact with the outer peripheral surface of the heating roller while winding the recording medium by an amount equivalent to a certain angle on the outer peripheral surface of the heating roller and heats the recording medium during the contact to dry ink.
The heating roller system heats the ink at about 100° C. to 200° C. at a heating roller part to evaporate not only water but also a solvent. To thus dry the solvent and the water, a necessary amount of heat is required to be supplied within a time during which the heating roller is in contact with a sheet. For this reason, when a recording medium having a high basis weight, which requires a large amount of heat, is used or when the conveying speed of a recording medium is increased, a plurality of heating rollers are required to be used. In this case, the arrangement of the heating rollers is set to a staggered arrangement, whereby apparatus size or sheet operability can be considered.
In a drying apparatus having a plurality of heating rollers arranged in a staggered manner, a solvent and water that have vaporized by the heating rollers arranged below move upward in an ascending air flow within the drying apparatus. In this case, the heating rollers arranged in the upper part are surrounded by an atmosphere of the solvent and the like that have vaporized by the lower heating rollers. Each of the heating rollers is equipped with a guide that guides a recording medium such that the recording medium will be conveyed without departing from a conveying path when the recording medium is loaded or conveyed. The solvent vaporizes as described above from within this guide. The vapor of the solvent can cause problems such as condensation on the guide and rusting the guide.
Apart from the prevention of adherence of solvent vapor to the guide, a technique is known that, in an image forming apparatus including a fixing unit that fixes unfixed toner images on a recording medium at least through heating, prevents water droplets from adhering to the recording medium after fixing (refer to Japanese Unexamined Patent Application Publication No. 2007-187821, for example).
The guide disclosed in Japanese Unexamined Patent Application Publication No. 2007-187821 includes wire members that extend at least in a direction having the component of the conveying direction of the recording medium and have a convex sectional shape protruding in a state having no edge against the conveying path side of the recording medium and a holder that supports the wire members so as to be arranged spaced apart from each other on a face along the conveying path.
The guide installed around a conventional heating roller is in a state in which the guide itself hinders the solvent and the water heated and vaporized by the heating roller from moving above within the apparatus. The outside of the guide is in a state directly receiving the vaporized solvent and the water ascending from around the heating roller arranged immediately therebelow. In this state, the solvent and the water condense on the inside and the outside of the guide owing to a temperature difference between the guide and the vapor.
Even when the wire members as in the guide disclosed in Japanese Unexamined Patent Application Publication No. 2007-187821 are used, a drying apparatus that supplies a necessary amount of heat for the heating roller used for the purpose of drying a continuous form normally installs a guide that covers a part in which a recording medium is wound around a conveying roller including the heating roller. When the continuous form is wound around the heating roller, humid air containing a vaporized solvent and water is inevitably generated from the winding part. This humid air is required to be exhausted from a drying chamber in which the winding part and the heating roller are installed.
In this case, if there is a conveying path through which the recording medium passes in a direction to which the humid air moves, the humid air generated below the conveying path adheres to the recording medium that again passes through the upper conveying path, thereby causing water droplets. In this case, even when the guide is formed of the wire members to attempt to exclude the humid air efficiently from the winding part, the attempt will not lead to a solution.